Method of exploding using reflective fractures



too otto United States Patent [72] lnventor Leon H. Robinson 3,066,733 12/1962 Brandon 166/299 Houston, Texas 3,075,463 1/1963 Eilers et al. 166/299 [2]] Appl. No. 775,388 3,266,845 8/1966 Williamson et al. 299/13X [22] Filed Nov. 13, 1968 3.464.490 9/1969 Silverman l66/247X [4S] Patented Oct. 13, 1970 E M A Ch. [731 Assignee Esso Production Research Company, W"

Houston Tex ASSLS/llnlE.tdI1l1llt!Illin A. Calvert e I A!!rne v r-.lames A. Reilly. John B. Davidson. Lewis H. a wrpmamn of Delaware Eatherton, James E. Reed and James E. Gilchrist [54] METHOD OF EXPLODING USING REFLECTIVE ifgggfifi ABSTRACT: A method for increasing the fluid conductivity of a subterranean formation disclosed in WhlCh the torma- [52] U.S. Cl 166/299; [ion is fractured by detonation f an Explosive within the {Oh OZ/219166508 mation. In the preferred embodiment. at least two wells are {5 ll Int. Cl... E2lb 43/26; drilled f the surface f the earth w the f r i and the F42d 3/04 formation is hydraulically fractured at both wells. An explol Field of 166/247- sive substance is injected into the fracture at one of the wells 308; 299/13- l6; l02/3U- 2L 331 75/2 and detonated. The formation lying between the two wells is fragmented by the energy of the detonation and the reflection [36] Reference? cued of the resulting shock wave from the fracture at the second UNITED STATES PATENTS well. In another embodiment of the invention, the well in 1,195,781 8/1916 Clark 299/13 which the explosive is placed need not be fractured. In still 1.237.063 8/1917 Kuhn.... 299/13 another embodiment, an explosive filled fracture and a spaced 1.978668 10/ l 934 Burg..... 102/2 l X reflective fracture are formed within a single well. Detonation 2.676.662 4/1954 Ritzmann 166/299 of the explosive causes high fragmentation in the zone 2,837,027 6/1958 Martin 102/2UX between the fractures.

l l l l l l l I ll] I ll ll l l l l l l l l i2222':a&ax-:-z 223w! METHOD OF EXPLODING USING REFLECTIVE FRACTURES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process utilizing wells in which an explosion is caused to occur. More particularly, the invention is related to processes for increasing the permeability or fluid conductivity of a subterranean rock formation. An explosive material is inserted into at least one well and detonated to cause fragmentation of the formation.

2. Description of the Prior Art Detonation of explosive substances placed within a well has long been used to increase the fluid conductivity or permeability of subterranean formations. This practice has been extensively employed in the oil industry to increase the productivity from oil-bearing formations. This technique has also been employed to increase the permeability of subterranean formations for recovering mineral values by in situ leaching of such formations.

These techniques have not been universally successful. This lack of success may be attributable to the fact that most of the energy of detonation is transferred to the rock as a compressive force. Since most hard formations can withstand considerable compression before failing, only a small area within the immediate vicinity of the wellbore is crushed by this process. Beyond this area, the shock wave has insufficient energy to cause compressive failure of the formation.

SUMMARY OF THE INVENTION This invention relates to a process for increasing the fluid conductivity of a subterranean formation. In the preferred embodiment, at least two wells are drilled into the formation, these wells are hydraulically fractured and an explosive substance is displaced into the fracture at one of these wells. The

explosive substance is detonated causing a shock wave to traverse the formation. Most of the energy of the shock wave is initially transferred to the rock, placing it in a state of compression. When the shock wave reaches the discontinuity in the formation presented by the fracture at an adjacent well, energy is reflected back toward the zone of detonation as a tensile wave. The sharp reversal in the state of the rock matrix from compression to tension causes fragmentation of the rock and increased fluid conductivity of the formation.

The primary object of this invention is to increase the fluid conductivity ofa subterranean rock formation.

Another object of this invention is to increase the fluid conductivity of a subterranean rock formation by drilling at least two wells into the formation and hydraulically fracturing the formation at these wells. One of the fractures is filled with an explosive substance which is detonated to create a shock wave which will traverse the formation. The resultant reflection of the shock wave from the fracture at the adjacent well will create a highly fragmented, rubble zone within the formation.

A further object of the invention is to increase the fluid conductivity of a subterranean rock formation by detonation of an explosive substance at two adjacent wells whereby the interference of the shock waves emanatingfrom each well fragments the formation and creates high permeability within the formation.

DESCRIPTION OF THE DRAWINGS FIG. I is a horizontal cross section of a subsurface formation.

FIG. 2 is a vertical cross section of a subsurface formation penetrated by a well.

DESCRIPTION OF THE PREFERRED EMBODIMENT Many subterranean oil-bearing formations have insufficient natural permeability to be economically produced. Vast quantities of oil may be stored in the interstices of the formation,

but lack of interconnecting pore spaces prevents the flow of the oil within the formation.

Other valuable minerals such as copper, uranium and phosphates are contained in formations which are too deeply buried to be recovered by conventional mining techniques. The only economical method for recovering such values would be in situ leaching using a solution which has the capability of extracting these metal values from the buried ore deposit. These minerals are often contained in rock which has an extremely low natural permeability to the injected fluids. This lack of natural permeability prevents injection and withdrawal of the leaching solution at rates which are economically attractive.

The process of this invention presents a method for increasing the natural permeability of subterranean formations which have oil or metallic minerals contained within the formation. The manner in which this process operates can be more easily understood by reference to the drawings.

Referring to FIG. 1. a subterranean formation containing the minerals of interest is shown generally at 10. Wells 11 are drilled from the surface of the earth and the formation is hydraulically fractured.

In a hydraulic fracturing treatment, fluid is injected down the well casing or tubing at rates higher then the rock matrix will accept. This rapid injection produces a build-up in well bore pressure until a pressure large enough to overcome compressive formation stresses and tensile rock stress is reached. At this pressure, failure occurs and a crack or fracture 12 is formed. Continued fluid injection increases the fractures length and width. In order to create a sharp reflective surface, the fracture should be relatively wide. To obtain a desired width, a large granular solid (usually sand) 13 is injected along with the fracturing fluid and deposited in the fracture.

In the vast majority of instances, the fractures formed will be vertical and oriented along a specific azimuth. Fractures grow in the direction that requires the least amount of work as indicated by the second law of thermodynamics. Since all subsurface rocks are under compressive stresses due to the weight of overburden, a fracture will orient itself such that it grows perpendicular to the axis of the smallest compressive stress. A vertical'fracture is formed when the vertical stress is larger than either of the horizontal stresses. Vertical fractures are produced in most formations since the vertical stress gradient is generally larger than the horizontal stress gradient.

The azimuth or orientation of vertical fractures in adjacent wells is generally parallel. The stresses imposed on a formation are a result of tectonic forces imposed on the formation over geological time. For a given geographical area, the horizontal stress will generally have a maximum value in one direction and a minimum value in another direction which is roughly perpendicular to the maximum stress. As a result of these stresses, the fracture will widen in the direction of minimum stress and extend along the line of maximum stress.

There are a number of methods of creating hydraulic fractures which are well known to those skilled in the art of oil production. The process of this application does not depend on the use of any particular method for the formation of such fractures.

Additional boreholes 14 are drilled into the formation and are subjected to hydraulic fracturing treatment to create fractures 15. The fracturing fluid is displaced by a pumpable, explosive substance 16 which then fills the fractures. It is preferred to employ a propping agent in the creation of the explosive fractures 15 as well as in the reflective fractures 12.

The explosive mixture 16 is detonated in any desired manner to create a shock wave which will emanate from the fracture l5 and travel through the formation. In the immediate vicinity of the explosive filled fracture 15, the formation will be highly fragmented due to compressive crushing of the rock. When the shock wave reaches the reflective fractures 12, it is reflected back toward the detonation zone. Immediately adjacent the reflective fracture, tensile slabbing may occur causing additional fragmentation of the rock matrix. However, most of the fragmentation occurs in the zone lying between the crushed zone 17 and the tensile slabbing zone 18.

The manner in which the process of this invention creates its high degree of fragmentation of a subsurface formation is not fully understood It is felt, however. that the result achieved can be explained by the mechanics of acoustics in a dense material.

Upon detonation of the explosive, an initial shock wave radiates from the area or point of detonation. It has been estimated that this initial shock wave transmits percent or less of the total blast energy to the rock matrix. Within the crushed zone 17 the energy of the initial shock wave is in excess of the compressive strength of the rock, and therefore this zone is highly fragmented. Beyond the crushed zone in the area 19, the energy of the initial shock wave has dropped below the compressive strength of the rock, and therefore no initial fragmentation occurs.

When the shock wave reaches the free face formed by the reflective fractures 12, a new physical phenomenon occurs. This physical phenomenon is known as impedance mismatch. The initial shock wave has been traveling through a material of high density, rock, and suddenly reaches the free face of the reflection fracture which is filled with a low density fluid, such as water or air, and interspersed propping agent, such as sand. The acoustic impedance of a material is proportional to the product of the density of the material and the velocity of sound in that material. The acoustic impedance of the rock matrix is relatively high; the acoustic impedance of the reflection fracture is much lower. Due to this sudden change in acoustic impedance-impedance mismatch-a sudden reven sal of forces occurs at the reflective fracture. It should be noted that the lower the density of the material in the fracture, the greater the impedance mismatch will be. It is therefore, preferable to displace the fracturing fluid with a lower density fluid, such as air, other gases, water or light hydrocarbons.

When the initial shock wave reaches the free face of the reflective fracture, the change in acousitcal impedance from the highly dense rock matrix to the less dense fracture system causes the rock to change from a state of compression to a state of tension. The tensile strength of the rock matrix is generally much less in absolute value than its compressive strength. This is particularly true in porphyritic ore deposits which have a natural system of fractures containing the minerals of interest. These fractures represent discontinuities in the rock matrix and sharply reduce the ability of the matrix to withstand tensile stress.

The reaction in zone 18 results in tensile slabbing. Due to the oncoming shock wave, the rock matrix is placed in a state of high compression. As the shock wave reflects from the free surface system, there is a sharp and sudden reversal of forces imposed upon the matrix and the creation of a tensile wave which reflects from the face of the fracture. This sudden change from high compression to tension causes the matrix to fragment as the imposed tensile stress exceeds the tensile strength of the rock.

As was previously stated, only 10 percent of the blast energy is consumed in the initial shock wave. The remaining energy is dissipated as heat and in the creation of the gaseous products resulting from the detonation. If the blast is well confined by proper tamping, as much as 70 percent of the available energy can be transferred to the rock. This energy will be transferred to the rock matrix as a compressive force which will follow behind the initial shock wave and probably is the primary source of energy for fragmentation in the stress relief zone 19, This compressive wave will also reflect from the reflective fracture surface and produce a corresponding tensile wave. As the state ofthe rock in the zone changes from compression to tension a high degree of fragmentation will result.

In certain geographical areas, hydraulically created fractures will have a tendency to orient in the horizontal, as opposed to the vertical, plane due to the stresses imposed upon the formation and planes of weakness in the formation. This will be particularly true in shallow formations where the verti cal stress component is low due to a relatively light over burden. ln formations where the fractures will orient in a horizontal plane. it is possible to practice the method of this invention in a single well.

As shown in FIG. 2 a well 20 is drilled into the formation of interest 21 and perforated at locations 22,23 and 24. These sets of perforations are isolated using straddle packers and are individually fracturedv Reflective fractures 25 and 26 are formed at the top and the bottom of the formation, respectively, and fracture 27 is formed near the center of the formation Fracture 27 is then filled with an explosive substance. Upon detonation of the explosive, the formation 21 will be highly fragmented in the area between the reflective fractures 25 and 26 in the manner previously described. It is preferred to employ one reflective fracture above and below the explosive filled fracture 27 to obtain the maximum utilization of the explosive force. However, in certain instances where the formation is extremely thick (over 100 feet) a single reflective fracture may, be employed.

In using the single well process it is necessary to confine the gaseous, detonation products in the central portion of the formation and prevent their entry into the reflective fractures. This can be conveniently done by placing cement plugs immediately above and below the detonation zone. The hole above the upper cement plug may be further tamped using conventional techniques.

Any suitable explosive may be employed in the practice of this invention. The explosive need only be in such a state that it can be displaced into the fracture system if so desired. The explosive may be a fluid such as desensitized nitroglycerin or a solid material suspended in a liquid carrier medium such as TNT in a liquid base.

The distance between the explosive filled fracture and the reflective fracture is determined primarily from the quantity of explosive which may be displaced into the explosive filled fracture, the maximum available energy of the particular explosive employed, and the powder factor which is the ratio of the weight of the explosive to that of the rock being blasted. These factors and their interrelationships are well known to those skilled in the art of blasting and need not be discussed in detail herein.

Generally, the fractures should be spaced by a distance of approximately 100 feet and the length of each fracture wing should be approximately 500 feet in length with an overall fracture span of 1,000 feet. Such a configuration will be satisfactory for use in most formations using desensitized nitroglycerin as an explosive.

While it is preferred to employ at least one reflective fracture containing no explosive for each explosive filled fracture, all fractures may be filled with explosive if desired. Such a system is generally not as economically attractive as the preferred embodiment, however, since generally more explosive per cubic yard of formation to be fractured must be employed. Furthermore, it is not essential that the explosive mixture be displaced within a fracture. It is possible to detonate the explosive charge within a wellbore and reflect the resultant shock wave from a reflective fracture in an adjacent wellbore. This method is normally less efficient than the preferred embodiment, however, since the shock wave will assume a radial configuration as opposed to the almost linear configuration which results from the preferred embodiment. Moreover, it may be possible to achieve a result which is satisfactory without fracturing either well when there is a high degree of natural fracturing within the rock matrix. When explosives are detonated in adjacent wells, the radiating shock waves will interfere with one another and result in excessive tensile stresses being imposed upon a large portion of the formatron.

lclaim:

1. An explosive method for increasing the permeability of a subterranean formation penetrated by a plurality of wells which extend from the surface of the earth to the formation comprising:

at. creating a reflective first fracture within the formation at a first well, said fracture being oriented to reflect an explosive wave in the formation; and

b. then, detonating an explosive at a second well in the formation which is remote from the first well to propagate an explosive wave through the formation to the reflective fracture, said explosive wave being reflected from the fracture toward the second well to create a zone of high fragmentation between the first and second wells.

2. A method as defined in claim 1 further comprising creating, prior to detonating the explosive, a reflective second fracture within the formation at a third well which is remote from the first and second wells and which is oppositely disposed to the first well with respect to the second well.

3. A method as defined in claim 2 further comprising, prior to detonating the explosive:

a. creating the first and second fractures by subjecting the formation to high fluid pressure;

b. displacing fracturing fluid and proppant into the fractures;

c. displacing the fracturing fluid from the fractures; and

d. filling the fractures with a fluid which is less dense than the fracturing fluid.

4. A method as defined by claim 2 wherein the first and second fractures are substantially vertical and substantially parallel.

5. A method as defined in claim 1 further comprising creating a fracture within the formation at the second well and injecting the explosive into the fracture prior to detonating the explosive.

6. A method for increasing the permeability of a subterranean formation penetrated by a plurality of boreholes comprismg:

a. injecting a high pressure fluid through one borehole into the subterranean formation to create a vertical, radiallyextending first fracture within the formation;

b. injecting a high pressure fluid through a second borehole into the formation to create a vertical, radially-extending second fracture within the formation which is substantially parallel to the first fracture;

c. displacing an explosive into the second fracture; and

d. detonating the explosive in the second fracture to create an area of high fragmentation between the first and second fractures in the subsurface formation.

7. A method as defined by claim 6 further comprising injecting. prior to detonating the explosive, a high pressure fluid through a third borehole to create a vertical, radially-extending, third fracture within the formation which is substantially parallel to the first and second fractures and opposite to the first fracture with respect to the second.

8. A method as defined by claim 7 wherein the high pressure fluid is removed from the first and third fractures and replaced by a substantially less dense fluid prior to detonating the explosive.

fl. An explosive method for increasing the permeability of a subterranean formation penetrated by a well which extends y from the surface of the earth to the formation comprising:

LII

a. creating a reflective fracture within the formation at the well at a first location, said fracture being oriented to reflect an explosive wave in the formation;

. displacing an explosive into the formation at a second location in the well which is vertically spaced from the fracture;

c. plugging the wellbore between the first and second locations', and then, detonating the explosive to propagate an explosive wave through the formation to the reflective fracture, said explosive wave being reflected from the reflective fracture toward the second location to create a zone of high fragmentation within the formation between the first and second locations.

10. A method as defined by claim 9 further comprising creating a reflective second fracture in the formation at a third location in the well which is vertically spaced from the first and second locations and which is oppositely disposed to the first location with respect to the second location and then plugging the wellbore between the second and third locations prior to detonating the explosive at the second location.

11. A method as defined by claim 9 wherein the explosive is displaced into a fracture in the formation at the second location. 

